1. Field of the Invention
The present invention is directed to scanning probe microscopes, and more particularly, a scanning thermal probe microscope employing a thermal probe as both a heat source and a temperature sensor for imaging and sensing thermal properties of a sample.
2. Description of Related Art
Modern materials science is increasingly concerned with the analysis and control of materials at a very small scale. Micro thermal analysis is directed to thermal analysis in conjunction with microscopy techniques using a thermal probe capable of providing thermal excitation. The use of microscopy allows small-scale regions of a sample to be selected for such imaging, for example. Micro thermal analysis is currently being used commercially to characterize various samples such as polymers, biological materials and electronic materials, among other types of samples. Notably, any local disturbance of the structure that results in a change in density, specific heat or thermal conductivity, can be detected by, for example, a thermal probe. One characteristic of particular interest is the glass transition temperature of a polymer, a key parameter in polymer technology. The behavior of a sample around the glass transition temperature is varied from rubbery or viscous to “glassy.” Being able to analyze polymers, and particularly their glass transition temperatures, under varying conditions and on a localized state, is of significant interest. Overall, the ability to make measurements such as calorimetric measurements on a localized scale, particularly on the nanometer scale, has been the subject of much development over recent years. The use of the scanning probe microscope has been critical in the development of this technology.
Scanning probe microscopes (SPMs), such as the atomic force microscope (AFM), are devices which typically use a sharp tip and low forces to measure the surface of a sample down to atomic dimensions. Generally, SPMs include a probe having a tip that is introduced to a surface of a sample to detect changes in the characteristics of a sample. By providing relative scanning movement between the tip and the sample, surface characteristic data can be acquired over a particular region of the sample and a corresponding map of the sample can be generated.
In an AFM, for example, in a mode of operation called contact mode, the microscope scans the tip or the sample, while keeping the force of the tip on the surface of the sample generally constant. This is accomplished by moving either the sample or the probe assembly up and down relatively perpendicularly to the surface of the sample in response to a deflection of the cantilever of the probe assembly as it is scanned across the surface. In this way, the data associated with this vertical motion can be stored and then used to construct an image of the sample surface corresponding to the sample characteristic being measured, e.g., surface topography. Similarly, in another preferred mode of AFM operation, known as TappingMode™ (TappingMode™ is a trademark owned by the present assignee), the tip is oscillated at or near a resonant frequency of the associated cantilever of the probe. The amplitude or phase of this oscillation is kept constant during scanning using feedback signals, which are generated in response to tip-sample interaction. As in contact mode, these feedback signals are then collected, stored and used as data to characterize the sample. Note that “SPM” and the acronyms for the specific types of SPMs may be used herein to refer to either the microscope apparatus, or the associated technique, e.g., “atomic force microscopy.”
Another type of SPM is the scanning tunneling microscope (STM). In an STM, similar to AFM, a probe having a tip is employed to scan a surface of a sample. However, in STM, the tip is conducting In operation, a current, known as the tunneling current, is made to flow between sample and the free end or apex of tip. This tunneling current is produced in response to a bias voltage applied between the sample and the tip and is sensitive to the tip-sample separation distance. During operation, maintaining a constant tunneling current through the use of the feedback loop thereby gives a generally constant separation of the tip above the sample surface. Similar to AFM, these feedback signals are indicative of a particular characteristic of the sample.
In sum, scanning probe microscopes are widely used for imaging very small objects down to the size of an atom. It provides powerful imaging ability, including sample characteristics relating to physical topography, force, capacitance, static charge and other parameters depending on its mode of use. The science of SPMs has expanded to allow its use for thermal analysis including mapping temperature, thermal conductivity, measurements of phase transition points, etc. One initial attempt to combine SPM and thermal analysis integrated a thermocouple on a metallic tip of the SPM. In this technique, as the tip approaches a sample it is cooled due to the heat transfer between the tip and the sample surface. An acquired temperature signal from the thermocouple is used as the feedback signal to measure, for instance, topography of a material. Since the feedback is used only to maintain the tip temperature constant it does not provide any measure of thermal parameters of the sample surface, it merely provides an alternate way of operating the SPM for non-conducting samples.
Next, in an attempt to measure true thermal parameters of a sample, another technique was developed based on the atomic force microscope, the scanning thermal probe microscope (SthM). In this case, a thermocouple which acts as a temperature sensor is placed at the apex of the AFM cantilever and conventional force feedback of an AFM, as described previously, is used to maintain contact between the tip and the sample. As a result, temperature distribution of a sample can be measured in addition to topography. However, its functions are limited in that it cannot measure thermal properties such as conductivity and phase transitions because it only constitutes a temperature sensor, i.e., it cannot heat a sample.
In yet another attempt to measure more thermal parameters, a scanning thermal probe microscope was developed that employs a resistive probe as both a heat source and a thermometer. Conventional force feedback of an AFM is used. The probe in this case operates at constant temperature while the applied power is monitored. In operation, because the more conductive a sample the more power required to maintain probe temperature, thermal conductivity can be measured by measuring the power used to maintain the probe temperature.
In all of the above-described systems, notably, the system cannot measure thermal properties of a sample that change with temperature. To further extend the capability of SThM, an improved system was developed in which localized thermal analysis was employed using a miniaturized resistive probe (system 20 shown in FIG. 2 and described below). Localized thermal analysis measures thermal properties of material by ramping the temperature of the probe, instead of using constant temperature, from which phase transition points, such as melting points, can be derived.
Turning initially to FIG. 1, a simplified diagram of a prior art resistive probe 20 having a tip 22 includes a resistive heater 24 at its tip which is used for thermal analysis. One example of such a probe is a Wollaston wire based probe (and thus is often referred to as a “Wollaston probe”) which, typically, has a coating of silver over a thin core of platinum. At tip 22 of the probe, the silver is etched away exposing the platinum core or filament. With this design, notably, almost all of the electrical resistance of the probe is located at tip 22. As a result, when an electric current is applied to the probe, only the tip heats. In operation, the electrical resistance of the probe also provides a measure of the temperature at the tip.
More particularly, resistive thermal probe 20 consists of three basic parts including a conducting wire 26, a tip 22 and a mirror 28. Tip 22 part has a much larger resistance than conducting wire 26. Mirror 28 is provided to detect the deflection of the probe when the probe interacts with a corresponding sample 30 during operation. Alternatives to the Wollaston probe have been explored by researchers in order to provide better spatial resolution. However, due to the difficulty to integrate a separate thermometer on a tiny tip, these alternatives are typically exclusively resistive probes, which serve as both a heater and a thermometer.
Overall, local thermal analysis is similar to a differential scanning calorimetry or modulated differential scanning calorimetry except it is for a small volume of material. In one known embodiment, DC current is used to raise the probe temperature and AC current is used for thermal modulation. Notably, the AC component of the applied heating input is primarily provided to improve thermal sensitivity, as its rate of change is faster than the DC heating component. This allows a more accurate measurement of power.
Overall, when performing thermal experiments, both components may be desired. Again, the slower DC heating makes it easier for the sample to reach equilibrium when performing the experiment, and the faster AC heating provides better sensitivity for thermal measurements. A schematic diagram of this system is shown in FIG. 2.
FIG. 2 illustrates control and sensing components of a localized thermal analysis instrument 50. Notably, instrument 50 employs two resistive probes 52, 54 to provide the thermal analysis: a reference probe 52 and a sample probe 54. The reference probe 52 is operated in open loop by the combination of a DC current and an AC current provided via a summing circuit 55. The same physical AC current also is applied to the sample probe in open loop. A comparator 56 and an integrator 58 form a feedback loop to maintain the DC voltage over the sample probe 54 following the reference probe 52. The difference between the two DC driving signals (60 and 62), provided by a comparator 64, serves as the circuit's DC output. The difference of AC voltage drops over the two probes is measured by a lock-in amplifier 66, whose amplitude and phase outputs serve as the circuit's AC output.
Using this technique, some phase transitions of materials, such as melting points, can be observed. However, one problem with this technique is that it does not provide reliable data. In particular, the system cannot separate the sample signal from contamination in the data caused by the probe itself. For example, when making polymer measurements, the signal from a probe can be tens of times larger than that from a sample. Despite the introduction of a reference probe and using simple subtraction of the voltages over the sample and reference probes, the problem is not obviated because the probe signal is not eliminated. Primarily, this is due to the fact that the thermal parameters, such as power, temperature, etc. do not have a linear relationship with respect to the measured voltage. As a result, the output is so heavily contaminated that its absolute value (i.e., using the reference probe) provides no useful information concerning the sample. Only abrupt changes in the data may indicate some phase transition in the material and even these signals need to be observed through derivatives of the output signals.
Another drawback of this latter system is that it does not employ a reliable scale of temperature, or provide any control thereof. Because the sample probe is placed on the surface of the sample during the measurement, while the reference probe is not, the sample probe needs to consume more power to keep its temperature following the reference probe. As the two probes are kept at the same voltage, the temperatures are necessarily different, thus injecting errors in the data. Moreover, this difference changes with the conductance of a sample. And, because the signal used to measure the temperature is changing (i.e., the heating current is variable) a true measure of temperature cannot be obtained. In addition, when a small AC current is used for heating, the effective noise associated with the temperature measurement is amplified, causing the data to be unreliable.
As a result, the field of localized thermal analysis was in need of a device that is capable of separating the sample signal from contamination introduced by the measuring probe. Moreover, a system that is able to provide a measure of temperature directly and continuously while in the process of heating the probe is also desired.